The invention relates to the field of optical heterodyne spectroscopy and more particularly to spectroscopic techniques using frequency-modulated light beams for probing the spectral properties of a sample.
Generally speaking, in optical spectroscopy a probe light beam of known frequency characteristics is directed at the sample under investigation, and one or more properties of the radiation from the sample are measured after interaction of the sample with the incident probe beam. From the observed properties information can be extracted concerning the spectral feature of interest. As advances are made in spectroscopic technique, an ever-increasing variety of quantities are observed which carry information about the spectral feature under examination. For example, in many spectroscopic arrangements the overall absorption and/or dispersion experienced by the probe beam is measured as the frequency of the probe beam is varied over a range including the spectral feature of interest. In other techniques more refined parameters such as higher-order susceptibilities are measured, and these may be determined through experimental arrangements for observing either steady-state properties of the sample or its transient response to a sudden change. In optical heterodyne spectroscopy the observable quantity carrying the information of spectroscopic interest is shifted to a frequency domain removed from that of the spectral feature under investigation, where the quantity can then be detected and analyzed more conveniently, more accurately, or more economically.
In an article entitled "Frequency-Modulation Spectroscopy: A New Method for Measuring Peak Absorptions and Dispersions" G. C. Bjorklund discloses an optical heterodyne spectroscopic technique in which a laser beam with an rf frequency modulation is used as a probe beam, and the desired spectroscopic information is contained in a beat signal at the modulation frequency. The Bjorklund method is also the subject of U.S. Pat. No. 4,297,035.
In particular, Bjorklund employs a single-mode laser beam having a frequency .omega..sub.c in the visible spectrum, which is modulated with a frequency .omega..sub.m, typically on the order of 500 megahertz, so as to produce a beam having first-order sidebands at frequencies .omega..sub.c .+-..omega..sub.m. In a typical spectroscopic experiment the modulated beam probes a sample having an absorption line in the vicinity of one of the sidebands, e.g., in the vicinity of the upper first-order sideband. Differential absorption of the two sidebands at frequencies .omega..sub.c .+-..omega..sub.m provides a measure of the absorption feature at frequency .omega..sub.c +.omega..sub.m with respect to the baseline established by the lower sideband at frequency .omega..sub.c -.omega..sub.m, which lies outside the frequency range of the absorption feature. When the modulated beam emerging from the sample is passed through a photodetector, a signal is produced at the beat frequency .omega..sub.m representative of the differential absorption, hence, of the spectral feature.
The success of the Bjorklund method depends upon the availability of a photodetector responsive to the frequency and power level of the beat signal at frequency .omega..sub.m. A lower limit is placed on the bandwidth of the photodetector by the modulation frequency .omega..sub.m, which at the minimum must be greater than the linewidth of the laser beam and greater than the width of the spectral feature of interest and, to derive full benefit from the Bjorklund technique, should be much greater than the width of the spectral feature.
Typical Doppler-broadened gases have linewidths on the order of 50 megahertz in the infrared portion of the spectrum and 2 gigahertz in the visible portion, whereas atmospheric pressure-broadened gases have even larger linewidths, on the order of 3 gigahertz in the infrared and 10 to 20 gigahertz in the visible. Thus, in making measurements on gaseous samples the modulation frequency .omega..sub.m, and consequently the minimum bandwidth of a suitable detector, must exceed at least 50 megahertz for analysis of Doppler-broadened lines in the infrared domain, and must exceed up to 20 gigahertz for measurements on atmospheric gases in the visible domain.
As is well known, increased photodetector bandwidth can be achieved only at the expense of sensitivity. It would be desirable, for example, to extend the Bjorklund technique to the 8 to 12-micron atmospheric wavelength window for observing numerous molecular species in the atmosphere known to have strong absorption features. However, suitably sensitive photodetectors having adequate bandwidth to handle the atmospheric pressure-broadened spectral features are not available in this frequency domain. Thus, the stringent requirements on bandwidth present a severe impediment to the extension of FM spectroscopy to this and other spectral regions of interest, as well as to applications in which optical power levels are constrained to be low.